Tetrachlorphthalonitriles, and in particular tetrachloroisophthalonitrile, are widely used for their important phytoprophylatic properties. Studies relative to improvements in the methods and processes which lead to the production of these important compounds are followed threfore with particular attention throughout the world.
The object of the present invention is to provide an improved process for the production of tetrachlorophthalonitriles from the corresponding phthalonitriles by vapour-phase chlorination in a fluidised bed, in a more simple manner and with less sophisticated and less costly apparatus than at the present time.
In this respect, the known fludised bed chlorination processes are generally carried out respecting the following conditions:
in the case of thermolabile organic substances, excessive heating temperatures for relatively long times are avoided so as to prevent the formation of decomposition compounds and polymerisation or limit these latter to very low levels;
the mixture comprising the chlorine and the substance to be chlorinated is prevented from reacting before entering the catalytic layer, so as to prevent uncontrollable premature reactions.
For these reasons, in known processes specified for chlorinating phthalonitriles (such as that described in U.S. Pat. No. 3,839,401) these compounds are fed not in the vapour phase but as molten liquids, and they are injected in an atomised state directly into the body of the catalytic layer, where the reaction chlorine is already present.
Moreover, in many of the known processes for chlorinating phthalonitriles (again such as in that of U.S. Pat. No. 3,839,401), the chlorinated product is recovered from the reaction gas mixture by using water or a solvent which is inert towards chlorine, in order to desublime and extract the reaction product.
It is apparent that each of these processes can be effected only by using apparatus which is sophisticated (required for example to atomise a molten liquid and distribute it in a fluidised bed) and costly (required for example to desublime and extract the product using water or chlorine-resistant organic solvents). Furthermore, transferring these processes to an industrial level poses considerable corrosion problems, problems of a toxicological, ecological and economical character, and plant operation problems.
The interest in simplifying the aforesaid known processes and in particular in simplifying the plant necessary to uprate them to an industrial scale and to enable them to operate is therefore apparent.
The studies of the applicant in this direction have resulted in an improved process which, in addition to attaining these objects, ensures tetrachlorophthalonitrile production at a purity exceeding 98%, this constituting a further important progressive step in comparison with the known art.